Many industries utilize dispensing systems to apply liquid material to a surface. In the motor vehicle industry, for example, liquid primers are applied to a perimeter region of a windshield as part of the process for installing a windshield in a vehicle body. These primers may be of various types and are preparatory to applying a urethane bead to the windshield that bonds the windshield to the frame. In a typical process, the windshield manufacturer applies a roughened black ceramic frit to a marginal or perimeter region of the windshield on the side of the glass that is bonded to the frame. Before the urethane bonding material can be applied to the frit, a first type of primer must be applied to the surface of the frit with a rubbing action. Often this first type primer is clear in color and is a surface activator that quickly evaporates and prepares the frit surface for application of a second type primer. In order to be effective, however, the clear primer cannot be simply applied but must be applied with a rubbing action as well.
After the clear primer is applied, a second type primer is applied to the frit, again with a rubbing action. Typically the second primer is black in color. The black primer improves adhesion of the urethane to the frit, but more importantly functions to block ultraviolet radiation that would otherwise cause degradation of the urethane. The black primer tends to have a higher viscosity than the clear primer, with the latter having a viscosity about that of water. A typical black primer may have a viscosity, for example, of about 40-100 centipoise.
The primers may be applied to the windshield by manual operations, but more commonly they are applied by dispensing the primer onto the frit using a liquid dispensing gun, and then applying a rubbing action by robotically moving a piece of felt across the frit by relative movement between the windshield and the gun. In a known process commonly referred to as “drip and drag”, the primer is applied onto the frit just ahead of the felt. The primer may be intermittently applied rather than continuously. When the type of the primer is changed, the felt must be changed or an opposite surface of the same felt piece used.
One such system utilizing a drip and drag process is disclosed in U.S. Pat. Nos. 5,277,927 and 5,370,905 which are owned by the assignee of the present invention, the entire disclosures of which are fully incorporated herein by reference. These patents provide a more detailed description of such apparatus and the various technical issues involved with applying these primers to a windshield.
Although the above referenced systems are a significant advance in the art, they as well as other known systems have limitations. For example, some known systems require manual changeover of the felt piece either for each type primer change or after each windshield is completed. This results in a substantial consumption of the felt material. Still further, most vehicle windshields are not flat panes of glass but rather are curved, some more than others, particularly near the marginal area or perimeter. This can cause a loss of or diminished contact between the felt applicator and the frit, especially as the robotic arm passes around corners. The drip and drag process can result in an uneven application, particularly of the more viscous black primer. The felt tends to push the primer ahead of it, thereby diverting primer to either side resulting in a “railroad track” profile in which the edges of the bead are thicker than the center region of the bead. Additionally, since the primers are applied by pulsing a dispensing gun on and off, it can be difficult to control the amount of liquid material applied to the frit.
Another system is described in U.S. Pat. No. 6,695,917 B2 issued to Wright et al and owned by the assignee of the present invention, the entire disclosure of which is fully incorporated herein by reference. The Wright system uses, in some embodiments, a dual rub concept wherein the rub blocks include respective dispensing nozzles and control valves. Fluid to be dispensed is supplied under pressure to the valve and nozzle. Quantity of material dispensed may be controlled for example by flow meters and flow regulators, or based on time duration that the valve is open to flow. However, accuracy of pressure based delivery systems is dependent on known viscosity characteristics of the fluid being dispensed.